Online acid barrage stacking anti-salt injection for capillary electrophoresis of 9-fluorenylmethylchloroformate-derivatized amino acids in high ionic strength solutions by UV detection

An acid barrage stacking (ABS) method has been shown to be feasible for online anti‐salt injection in CE of 9‐fluorenylmethyl chloroformate (FMOC)‐labeled amino acids (AAs) detected by common UV absorption. The operation was performed on normal polar CE by sucking in an extra plug of acid following a sample zone, serving as a selective acid barrage to block the backward migration of weak anionic analytes due to a sudden mobility reduction via acid‐base reaction which does not affect strong co‐ions such as Cl− to penetrate the barrage freely. By CE‐UV of FMOC‐AAs in various NaCl solutions, the effectiveness of ABS was firmly validated, able to stand up to 500 mM NaCl and to stack analytes by 103‐fold calculated from the UV detection limits, that is 0.01 µM for ABS and 10 µM for non‐stacking injection. The method was also validated by determining trace Glu and Asp in real samples of rat brain microdialysate, rat serum and human saliva. The intraday RSDs were 0.33–4.9% for migration time and 1.8–9.6% for peak area. The recoveries measured by spiking technique were 82–115% for Glu and 86–116% for Asp. Working equations were obtained by plotting peak height vs. concentration at 0.1–50 µM, with correlation coefficients of >0.999. The contents of Glu and Asp were thus found at 0.26–0.83 µM and 0.24–0.64 µM respectively, in rat brain microdialyste; 37–40 µM and 8.4–10 µM, respectively, in rat serum; and 3.5–5.8 µM and 1.0–4.1 µM, respectively in human saliva. They were consistent with the data from other methods.